The research is directed towards a clearer understanding of the delays and deficits in the syntactic development of retarded and autistic individuals. Research has demonstrated that some retarded and probably all autistic children are significantly more delayed in language development than would be predicted by their overall cognitive level. The purpose of this research is to investigate whether this significant language delay is related to the process by which these children acquire syntactic rules. Specifically, it is hypothesized that some retarded and all autistic children are particularly deficient in utilizing semantic constraints in the development of grammatical forms which are known to facilitate syntactic development in normal children. This hypothesis is derived from past research demonstrating that these children have difficulty making effective use of semantic information in cognitive tasks. The hypothesis will be addressed in two studies that are designed to investigate retarded and autistic children's ability to use semantic constraints in the acquisition and use of a number of grammatical constructions. In one study five retarded and five autistic children will be followed longitudinally, collecting regular samples of their spontaneous speech. The transcripts will be analyzed for the syntactic-semantic relationships in their utterances. The ability to use semantic information effectively in the acquisition of certain syntactic rules will be related to the amount of discrepancy between the subjects' mental age and language level. The second study is cross-sectional in design. Mentally retarded, autistic and normal children will participate in a series of experiments designed to test their comprehension and elicited production of a number of grammatical forms in a variety of semantic contexts. Again, the ability to make effective use of semantic constraints to understand or produce a particular form will be related to the degree of discrepancy between mental age and language level. The results from these studies will lead to an understanding of how different retarded and autistic children process various syntactic rules and to what extent their syntactic development follows the same route as in normal children. The findings will lead to the development of better diagnostic and screening instruments for language impairments, and training programs for teaching syntactic rules to language delayed children.